Optimize duplicate transaction input check.

Profiling showed the duplicate transaction input check was taking around
6% of the total CheckTransactionSanity processing time.  This was largely
due to using fmt.Sprintf to generate the map key.

This commit modifies the check instead to use the actual output as a map
key.

The following benchmark results show the difference:

Before: BenchmarkOldDuplicatInputCheck    100000     21787 ns/op
After:  BenchmarkNewDuplicatInputCheck   2000000       937 ns/op

Closes #2